In the field of electronics, multilayer semiconductor structures are frequently used.
A particular example of such structures is a structure of the semiconductor-on-insulator (SeOI) type.
A structure of the SeOI type typically comprises from its base to its surface a supporting substrate, an electrically insulating layer and a thin semiconducting layer, called an active layer, in or on which electronic components are generally intended to be formed.
When the thin layer is in silicon, the structure is designated by the term of SOI, an acronym of “Silicon-On-Insulator.”
The electrically insulating layer is in a dielectric material, notably an oxide of the material of the supporting substrate and/or of the thin layer. This layer is then usually designated by the term of BOX, acronym of “Buried Oxide.”
Recently, SOI structures having an extra-thin layer of silicon have been developed.
These structures are designated by the term of “FDSOI,” for “Fully Depleted SOT,” i.e., a totally depleted SOT.
By “ultra-thin” is meant a thickness of less than or equal to 50 nm, preferably less than or equal to 12 nm, which may even be reduced down to about 5 nm.
FDSOI structures are particularly advantageous for producing planar electronic components, for example, FDMOS (acronym of “Fully Depleted Metal Oxide Semiconductor”) transistors, for which the channel is formed in or on the thin layer of silicon.
Because of the extreme thinness of the thickness of the thin layer, the threshold voltage of the transistor (usually noted as Vt), which depends on this thickness, is very sensitive to thickness variations of the thin layer.
For such applications, optimum uniformity of the thin silicon layer is, therefore, sought so as to have minimum Vt variability from one transistor to the other.
Considering the small dimensions of these devices and their great proximity, it is necessary to measure the thickness variation between points very close to each other, for example, every 0.5 μm.
This implies, during the method for making the SOT, measuring the thickness of the thin silicon layer and of the electrically insulating layer, in different points of the surface of the SOT, in a wide range of spatial wavelengths, typically comprised between 0.5 μm and 300 mm.
Present measurement methods are based on optical measurements, notably ellipsometry or spectral reflectometry.
In both cases, these methods imply conducting a large number of measurements by illuminating the SOT with a light flux having several optical wavelengths, so as to not only measure the thickness of the silicon layer but also that of the buried oxide layer.
However, with these techniques, it is not possible to conduct measurements with spatial wavelengths as small as 0.5 μm.
Thus, an ellipsometer allows measurements to be conducted with spatial wavelengths greater than or equal to about 40 μm.
On the other hand, these measurements take a long time and are a penalty in the manufacturing cycle of the SOIs.
Moreover, measurements conducted by ellipsometry or reflectometry with a single optical wavelength would not give the possibility of determining with sufficient accuracy the thickness of the silicon layer, since, for a given optical wavelength, the measured thickness of the silicon layer depends on the thickness and on the nature of the underlying buried oxide layer.
Document WO 2014/072109 discloses a method for measuring thickness variations in a layer of a multilayer semiconductor structure (typically an SOI, in particular, an FDSOI) intended to conduct measurements, in particular, in the range of spatial wavelengths of between 0.5 and 40 which is not accessible with the above-described measurement methods.
This method comprises:                acquiring, with at least an image acquisition system, at least one image of the surface of the structure, the image being obtained by reflecting a quasi-monochromatic light flux on the surface of the structure,        processing at least one acquired image so as to determine, from intensity variations of the light reflected by the surface, the variations of the thickness of the layer to be measured (typically, the thickness of the superficial silicon layer),        wherein the wavelength of the quasi-monochromatic light flux is selected so as to correspond to a minimum of the sensitivity of the reflectivity of the multilayer structure with respect to a layer of the structure other than the layer for which thickness variations have to be measured.        
The sensitivity of the reflectivity with respect to a layer of the structure, which is homogeneous to the reciprocal of a length, is defined as being the ratio between:                the difference between the reflectivities of two multilayer structures for which the considered layer has a given thickness difference from one structure to the other (for example, 0.1 nm) and        the given thickness difference,        
the thicknesses of the other layers as for them being identical in both structures.
FIG. 1 illustrates the curves of the sensitivity of the reflectivity (noted as SR, expressed in Å−1) of an FDSOI structure versus the wavelength λ with non-polarized light calculated relatively to the silicon layer (curves Si1 and Si2) and relatively to the buried oxide layer (curves BOX1 and BOX2), for a thickness variation of the considered layer of 0.1 nm.
In this structure, the silicon layer has a thickness of about 12 nm and the buried oxide layer has a thickness of about 25 nm.
The rectangle in dotted lines appearing on this graph indicates an optimum range of wavelengths for illuminating the structure and for acquiring an image of the reflected light in order to determine the thickness variations in the silicon layer.
Indeed, in this interval, the sensitivity of the reflectivity with respect to the buried oxide layer is, in absolute value, a minimum (curves BOX1 and BOX2 passing through 0).
This means that the reflectivity variation measured with a quasi-monochromatic light flux in this range of wavelengths (being expressed, on an image of the surface, by intensity variations of the pixels) essentially depends on the thickness variations of the silicon curve to be measured.
It is, therefore, possible to determine, from intensity variations of the light reflected by the surface of the structure, the thickness variations of the layer to be measured.
In the illustrated example, the optimum wavelength of the light flux is comprised between about 510 and 530 nm. A quasi-monochromatic interferential filter around 515 nm may, therefore, be selected for forming the incident light flux.
In order to build a map of the thickness variations of the layer, a calibration curve is first computed to establish a relationship between the grey levels of the acquired image and a local thickness of the layer to be measured.
An example of such a calibration curve is shown in FIG. 2.
The computation of such a curve requires:                on the one hand, measuring (e.g., by ellipsometry) the thickness of the silicon layer of several FDSOI structures corresponding to given product specifications (such specifications typically comprise a target thickness of the silicon layer, e.g., 12 nm, and a target thickness of the buried oxide layer, e.g., 25 nm);        on the other hand, acquiring images of zones of the surface of each of the structures with a quasi-monochromatic incident flux having a given wavelength, and measuring the intensity of the pixels of the images.        
It is thus possible to associate a determined thickness of the silicon layer with a corresponding grey level in the image.
The measurement points allow constructing a theoretical curve (c) that comprises, in abscissa, the grey levels GS (arbitrary unit), and in ordinate, the thickness t of the superficial silicon layer (angstroms).
This calibration curve is then used during inspection of each structure corresponding to the product specifications for which the curve has been computed.
During inspection of a structure, a zone of the surface of the structure is illuminated with the above-mentioned quasi-monochromatic light flux and an image of the light flux reflected by the zone is acquired.
The intensity of the pixels of the image is measured and, thanks to the above-mentioned calibration curve, the thickness of the silicon layer in this zone is deduced.
However, for an FDSOI structure corresponding to a product with given specifications, the thickness of the buried oxide layer is also likely to vary within the same structure and/or from one structure to another one.
As shown in FIG. 1, a thickness variation of the buried oxide layer corresponds to a different wavelength to be applied, which, in turn, induces a variation of the reflectivity of the silicon layer and thus uncertainty on the real thickness of the thickness of the layer.
Such uncertainty is considered to range from 10% to 15% for an FDSOI structure corresponding to given product specifications.
The users of FDSOI structures now have increasingly stringent requirements regarding the accuracy of the mapping of the thickness variations and it thus becomes necessary to reduce the measurement error so as to reach an error of 1% or less.
To improve the situation, one possibility would be to determine, for each measurement zone, a specific wavelength corresponding to the minimum of the sensitivity of the reflectivity for the respective thickness of the buried oxide layer, and to use a quasi-monochromatic light flux having the specific wavelength to illuminate the structure in this zone.
However, such a solution is not practical on an industrial scale since it would require a high number of filters to obtain the quasi-monochromatic light flux with the desired wavelength.